Because a poet puts a particular truth before the world, it does not
necessarily follow that he has known or worked out all its great
consequences or that having done so, he is able always to express them
fully.
— "The Gita according to Gandhi" p. 9

I get lost trying to get its meaning, starting from the meaning of "put[ing] a particular truth before the world".

Because a poet expresses some truth in a way for the whole world to read and consider...

Next we have:

it does not necessarily follow that he has known or worked out all its great consequences

This simply means:

just because he's managed to write it down into words, that doesn't mean that he has necessarily fully understood it...

And lastly:

or that having done so, he is able always to express them fully.

Here, having done so refers to the poet fully understanding the truth he is writing about. So, the meaning of this last part is:

and even if the poet fully understands this truth, that doesn't ensure his words will be able to completely express the full profundity of it.

Put that all together, and here is my paraphrase:

Just because a poet has written about some truth doesn't mean the poet has managed to fully understand that truth. And even if he has managed to fully understand it, that doesn't mean his poetic words will unlock all the mysteries associated with it.

The general thrust of this sentence is roughly "placing an extremely high value on or advocating for some piece of knowledge does not guarantee the ability to fully understand or properly express that knowledge". It's presented as an aphorism, generalized to a prototypical poet. Good poets are able to express things very well, and (I presume this is so in the author's cultural context) assumed to be very intelligent. The statement conveys the idea that everyone is susceptible to these mistakes, because even someone as wise and eloquent as a poet can make them.

A breakdown:

1) Because a poet puts a particular truth before the world

Adding just to the beginning or changing because to even though will make the meaning of this clause more clear. It sets the stage which the rest of the saying is contrasted against. Puts before means values higher than. This is idiomatic; no one can literally physically place either an abstract concept or the entire planet somewhere. The poet values some truth or piece of knowledge more than the entire world; this is poetic license used to show just how strongly the poet cares about that truth. It's extremely unlikely (though possible) that someone would value a particular fact above everything else.

Credit to JR for mentioning another possible meaning of put before the world: to express the idea for everyone to hear, metaphorically placing the truth in front of the entire world so that it may be examined and understood. This meaning is at least as likely as my initial interpretation.

2) it does not necessarily follow that

Even if what was just said (1) is true, what follows (3 and 4) may be false.

3) he has known or worked out all its great consequences

(This should be read as a potentially false statement) The poet fully understands the meaning and effects of the truth.

4) or that having done so, he is able always to express them fully

Or that having done so tells us to assume that 3 is true (remember, the overall point is that 3 and 4 are very possibly false). The meaning here is that the poet fully understands the truth and is able to correctly and completely explain it to others.

Putting it all together, we can parse it like so:

A poet values some truth more than anything else; or, a poet expresses some truth to as many people as possible. (This is the premise; it gives us our starting point and context for the rest of the passage.)

Understand that just because 1 is true...

It is not guaranteed that the poet completely understands that knowledge.

Even with perfect understanding of the truth (i.e. even if 3 is true), it is still not guaranteed that the poet can completely and correctly explain it to anyone else.

This phrase makes an attempt to illustrate the limits of written communication.

The context of the first phrase is relegated to the simple event of a poet saying something. Either by design or by chance the statement is true.

The rest of the text notes that having simply uttered words that are true, doesn't by necessity mean that the poet saying them knows, or expected them to be true. Of course, even if the poet thought it was true, he may have known only a part of the truth, without understanding all of it.

However, if the poet did in fact know what he was saying was true with scholarly accuracy, he might not be able to describe it adequately.

Take the following statement as an example:

I have gold and its shiny!

By this simple statement we cannot determine how much the author knows about gold. He may have exhausted all of his knowledge on the subject in those six words. But the possibility exists that he also knows that the current exchange rate is roughly $1245/ounce, or that its elemental symbol is Au originating from the Latin word for gold, aurum, or that is melts at nearly 1948 degrees Fahrenheit or its atomic weight is roughly 197.

Even those few facts leave out a wealth of knowledge about malleability, ductility, resistance to corrosion, electrical conductivity, etc.

To often we judge statements as being more, or less then they were intended. It is impossible to truly know the intent of another mind.

Just because a poet puts a particular truth before the world, it is not reasonable to assume that he himself experienced the truth or all the consequences of the truth, and at the same time it's also not right to assume that he can express the truth fully.

It has a simple form - Because the poet did it, the fact doesn't necessarily follow A or B

The fact = The poet puts the truth before the world. Or in a simple form, the poet says/writes the truth before the world.

A = The fact does not necessarily follow that he has known or worked out all its great consequences

B = that having done the fact, he is able always to express them fully.